1. Field of the invention
The invention relates to metal ladders.
2. Description
The stiles of metal ladders are, in general, tubular, with a closed profile, in order to impart to them a high resistance to lateral forces. However, such stiles have to be provided with openings for the passage of rungs and the openings are difficult to machine. Furthermore, it is well known to the man skilled in the art that tubular profiles are heavier than open profiles.
Ladders are also known of which the stiles are constituted by simple open profiles of C or I section. These ladders are easy to manufacture, and less heavy then those of which the stiles are of closed profile. However, the connection between the stiles and the rungs gives rise to a weak joint, the web of the profiled members forming the stiles, on which the rungs are fixed, readily deforms under the action of forces to which the ladder is submitted when in use. In particular, the ladder deforms laterally when the user is at mid-height, which can give rise to a feeling of insecurity.
In order to overcome this disadvantage, it has been proposed in French Pat. No. 1,501,748 to reinforce the stiles at the region of each of the ends of the rungs, by a reinforcing plate secured to the stile by the formation at the edges of each reinforcing plate of flanges which are engaged by longitudinal flanges of these stiles or surround the longitudinal flanges, the outer flanges being inserted into the inner flanges.
However, the various reinforcing plates must be threaded at one of the ends of the stile, which is time-consuming and costly. When the stiles are of treated aluminum, there is a risk of cracking during the engagement of the flanges, which may cause dangerous starting points for complete failure. In addition the reinforcing plates are poorly secured longitudinally. In order to prevent the sliding of the reinforcing plates, provision is made in U.S. Pat. No. 1,501,748 for the machining on the surrounding flanges, bosses which act to deform the flanges which are engaged; but these bosses are of little value and may give rise to cracking of the stiles.
The present invention has for its object a ladder of which the rungs are of open profile, and reinforced by reinforcing plates in the region of each of the ends of certain of the rungs at least, and in which the reinforcing plates are adequately secured.